A hydrogen storing alloy is an alloy that reacts with hydrogen to form a metal hydride, and since the alloy can reversibly store and release a large amount of hydrogen in the vicinity of room temperature, practical utilization thereof as a battery material has been advanced in various fields, such as: a nickel-hydrogen battery (also referred to as a “Ni-MH battery”) mounted on an electric vehicle (EV), a hybrid electric vehicle (HEV: a motor vehicle using two power sources of an electric motor and an internal combustion engine in combination), and a digital still camera; and a fuel cell.
Various alloys, such as an AB5 type alloy typified by LaNi5, an AB2 type alloy typified by ZrV0.4Ni1.5, an AB type alloy, and an A2B type alloy, are known as a hydrogen storing alloy. Many of these alloys are composed of a combination of an element group that has high affinity with hydrogen and plays a role of increasing hydrogen storage capacity (such as Ca, Mg, rare earth elements, Ti, Zr, V, Nb, Pt, and Pd) and an element group that has a relatively low affinity with hydrogen and a small hydrogen storage capacity but plays a role of promoting hydrogenation reaction and reducing the reaction temperature (such as Ni, Mn, Cr, and Fe).
Among them, an AB5 type hydrogen storing alloy having a CaCu5 type crystal structure, for example, an alloy using Mm (misch metal) which is a rare earth mixture in the A site and elements such as Ni, Al, Mn and Co in the B site (hereinafter, this type of alloy is referred to as a “Mm-Ni—Mn—Al—Co alloy”) has features that a negative electrode can be composed of relatively inexpensive materials compared with other alloy compositions, and that a closed-type nickel-hydrogen battery with long cycle life and little internal pressure increase due to gases generated during overcharging can be composed.
With respect to this type of AB5 type hydrogen storing alloy, for example, Patent Literature 1 (WO 2006/085542) discloses a low Co hydrogen storing alloy having a CaCu5 type crystal structure that can be represented by the general formula MmNiaMnbAlcCod (wherein Mm represents a misch metal, 4.0≦a≦4.7, 0.30≦b≦0.65, 0.20≦c≦0.50, 0<d≦0.35, and 5.2≦a+b+c+d≦5.5), wherein the CaCu5 type crystal structure has a crystal lattice having an a-axis length of 499.0 pm or more and a c-axis length of 405.0 pm or more; and in a pressure-composition isothermal chart (PCT curve) at 45° C., equilibrium hydrogen pressure at a hydrogen storage capacity (H/M) of 0.5 is 0.06 MPa or less.
Patent Literature 2 (WO 2007/040277) discloses a hydrogen storing alloy having a CaCu5 type crystal structure that can be represented by the general formula MmNiaMnbAlcCodFee (wherein Mm represents a misch metal including La, 0.2≦d≦0.5, 5.025≦a+b+c+d+e≦5.200), wherein the content of La in the hydrogen storing alloy is 13 to 27 wt %; and the CaCu5 type crystal structure has a lattice volume of 88.70×106 (pm3) or less and a full width at half maximum at the (002) plane of 0.29(°) or less, these values being obtained by subjecting the alloy to X-ray diffraction measurement and elaboration of a lattice constant.
Patent Literature 3 (WO 2008/123616) discloses a hydrogen storing alloy comprising a matrix phase having a CaCu5 type crystal structure, wherein when the alloy is subjected to point analysis with an energy dispersive X-ray analyzer (EDX), the Fe peak intensity ratio which is the ratio of the Fe peak intensity of a segregation phase to the Fe peak intensity of a matrix phase [[(Fe peak intensity of a segregation phase)/(Fe peak intensity of a matrix phase)]×100(%)] is 103(%)<Fe peak intensity ratio<245(%).
Further, Patent Literature 4 (Japanese Patent Laid-Open No. 2011-231362) discloses an Al-containing hydrogen storing alloy of a CaCu5 type crystal structure having a space group of International Table Number 191 (P6/mmm), wherein in the crystal structure analysis of the hydrogen storing alloy, a Beq ratio (2c/3g) as a ratio of a thermal vibration parameter Beq (2c) at a 2c site to a thermal vibration parameter Beq (3g) at a 3g site is 1.4 to 10.0.
When it is taken into consideration that a hydrogen storing alloy is used for a Ni-MH battery to be mounted on an electric vehicle, a hybrid electric vehicle, and the like, it is necessary to further improve the charge-discharge cycle ability and the output characteristics of the battery.
Particularly, in the Ni-MH battery using a hydrogen storing alloy as a negative electrode active material, the corrosion of the hydrogen storing alloy gradually proceeds by a strongly alkaline electrolyte solution. Therefore, it is necessary to improve the corrosion resistance of the alloy for achieving the longer life-span of the battery. The corrosion of the alloy proceeds for the following reasons: That is, a hydrogen storing alloy having a surface state that is rich in nickel has high initial degree of activity. However, manganese, aluminum, and the like in the hydrogen storing alloy are oxidized by oxygen generated from the positive electrode and an alkali electrolyte solution to form a hydroxide, and the hydroxide produces a film poor in conductivity on the alloy surface to reduce the negative electrode conductivity. Thus, the corrosion of the hydrogen storing alloy proceeds by the electrolyte solution.
Therefore, as a method for improving the corrosion resistance of a hydrogen storing alloy, for example, Patent Literature 5 (Japanese Patent Laid-Open No. 9-63581) proposes a method including bringing a hydrogen storing alloy electrode into contact with an alkaline aqueous solution to increase the specific surface area of the hydrogen storing alloy powder and then bringing the resulting hydrogen storing alloy electrode into contact with an acidic aqueous solution having a pH of 3 to 6 which contains fluorine ions to fluorinate the surface of the hydrogen storing alloy powder having the increased specific surface area.
On the other hand, since the output characteristics of a Ni-MH battery is mainly influenced by the discharge characteristics of a metal hydride electrode, in order to improve the high rate discharge characteristics of the metal hydride electrode, there is proposed a method of previously immersing a hydrogen storing alloy powder in a high-temperature aqueous alkali solution to thereby activate the hydrogen storing alloy powder. For example, there is disclosed a method of surface-treating a hydrogen storing alloy powder with a weakly acidic aqueous solution having a pH value of 0.5 to 5 (for example, refer to Patent Literature 6 (Japanese Patent Laid-Open No. 11-260361)).
Further, in order to increase the initial activity of a hydrogen storing alloy, the hydrogen storing alloy is subjected to surface activation treatment with an alkali aqueous solution or the like to form a nickel-condensed layer acting as a hydrogen catalyst layer in the vicinity of the surface of the hydrogen storing alloy to facilitate the hydrogen storage and release of the hydrogen storing alloy. For example, Patent Literature 7 (Japanese Patent Laid-Open No. 2002-256301) discloses a hydrogen storing alloy in which a nickel-condensed layer is formed in the vicinity of the surface of the hydrogen storing alloy, and the Ni magnetic susceptibility is 3 to 9 as a quantitative definition of the nickel-condensed layer.
As described above, a hydrogen storing alloy powder has been immersed in an alkali solution to perform alkali treatment for the purpose of increasing both the charge-discharge cycle ability and the output characteristics of a battery. When the hydrogen storing alloy powder is subjected to alkali treatment, alloy components, particularly Mn, Al, and Mm (La, Ce, Nd, Pr), are eluted from the alloy surface into the alkali solution, and components essentially comprising Ni, Co and Fe which are the remaining alloy components form the surface layer of the hydrogen storing alloy. Therefore, Ni, Co, and Fe which are ferromagnetic substances will be present on the surface of the hydrogen storing alloy, and the residual magnetization tends to be high. That is, in the conventional research and development of the hydrogen storing alloy, the research and development have generally been conducted in the direction of increasing the residual magnetization of the hydrogen storing alloy.
[Patent Literature 1]
International Publication No. WO 2006/085542
[Patent Literature 2]
International Publication No. WO 2007/040277
[Patent Literature 3]
International Publication No. WO 2008/123616
[Patent Literature 4]
Japanese Patent Laid-Open No. 2011-231362
[Patent Literature 5]
Japanese Patent Laid-Open No. 9-63581
[Patent Literature 6]
Japanese Patent Laid-Open No. 11-260361
[Patent Literature 7]
Japanese Patent Laid-Open No. 2002-256301
[Patent Literature 8]
Japanese Patent Laid-Open No. 2010-255104